There is shown schematically in FIG. 1 an elevational view of a first-generation tethered airfoil wind energy conversion system, which was designed by the present inventor and which is included here for illustrative purposes. Referring to that FIGURE, the following derivation is made.
Consider the tethered airfoil wind energy conversion system (hereinater "TAWECS") of FIG. 1 to be a large circular airfoil of planform surface area S, and wind passing over it produces lift L. The portion of L that is effective in reducing the air pressure over the TAWECS vent (in which is mounted a conventional wind turbine and electric generator) should be (A.sub.v /S)L, where A.sub.v is the area of the vent. Then the RMS decrease in air pressure over the TAWECS vent caused by the wind, EQU .DELTA.p=(A.sub.v /S)L.noteq.A.sub.V =L/S (lb/ft.sup.2)
The RMS head due to wind input to the TAWECS, EQU h=.DELTA.p/.rho.=L/S.rho. (ft)
where .rho. is the air density (0.0023 slug/ft.sup.3). From O. W. Eshbach, "Handbook of Engineering Fundamentals", 2nd Ed., Wiley, 1952, p. 7-75 to 7-77, EQU C.sub.L =L/(1/2.rho.V.sub.w.sup.2 S)=(3/20)a
where
C.sub.L =coefficient of lift PA1 V.sub.w =RMS velocity of wind over the airfoil surface (ft/sec) ##EQU1## Then EQU L=3/40 a.rho.V.sub.w.sup.2 S (lb) EQU h=3/40 a .rho.V.sub.w.sup.2 =.DELTA.p/.rho. (ft)
The above derivation, which will be useful in the detailed description to follow, does not provide for the local variations in pressure over the airfoil surface. Instead, these calculations were for gross, first order feasibility estimates, using overall or average airfoil properties, and RMS values for the time-varying parameters. (A more detailed analysis using partial differential equations would be required to predict local pressure variations.)
In literature searching prior to the preparation of this document, the present inventor has become aware of the following prior art:
Prior U.S. Patents
______________________________________ Patentee Pat. No. Issue Date ______________________________________ Bolie 4,017,205 April 12, 1977 Carson, et al 4,018,543 April 19, 1977 Bolie 4,116,581 Sept. 26, 1978 ______________________________________
Bolie, in U.S. Pat. No. 4,116,581, has described a wind turbine (hereinafter "WT") employing airfoil principles to enhance the pressure drop across the WT, i.e., reduced pressure on the suction side. The Bolie structure employs a fixed hemispherical shell containing the WT in the lower half of the structure, and a rotating hemispherical shell in the upper half, with a vent oriented into the wind, and curved deflectors to direct the wind into the WT. The Bolie structure is rigid, implying size limitation, dictated by strength of materials considerations, on WT radii of probably 50 feet or less. Since the Bolie structure is approximately a sphere, with two opposing suction sides and camber of approximately 0, the reduced pressures on the top and bottom hemispheres should approximately cancel each other, leaving only the wind flowing through the aperture being effective in driving the WT, thus providing no aerodynamic advantage over a conventional horizontal axis WT, and a considerable economic disadvantage.
In both of the Bolie patents, the angle of attack of the airfoil .alpha.=0; an angle of attack .alpha.&gt;0 is impractical with the Bolie embodiments. Further, the Bolie structure in U.S. Pat. No. 4,116,581 must rotate into the wind, implying mechanical wear problems.